Multiprotocol label switching (MPLS) is a scheme in high-performance telecommunication networks which directs and carries data from one node to the next node. In MPLS labels are assigned to data packets. Packet forwarding decisions from one node to the next node in a network are made based on the contents of the label for each data packet, without the need to examine the data packet itself.
Generalized Multiprotocol Label Switching (GMPLS) is a type of protocol which extends multiprotocol label switching to encompass network schemes based upon time-division multiplexing (e.g. SONET/SDH, PDH, G.709), wavelength multiplexing, and spatial switching (e.g. incoming port or fiber to outgoing port or fiber). Multiplexing, such as time-division multiplexing is when two or more signals or bit streams are transferred over a common channel. In particular, time-division multiplexing (TDM) is a type of digital multiplexing in which two or more signals or bit streams are transferred as sub-channels in one communication channel, but are physically taking turns on the communication channel. The time domain is divided into several recurrent timeslots of fixed length, one for each sub-channel. After the last sub-channel, the cycle starts over again. Time-division multiplexing is commonly used for circuit mode communication with a fixed number of channels and constant bandwidth per channel. Time-division multiplexing differs from statistical multiplexing, such as packet switching, in that the timeslots are returned in a fixed order and preallocated to the channels, rather than scheduled on a packet by packet basis.
The optical transport hierarchy (OTH) supports the operation and management aspects of optical networks of various architectures, e.g., point-to-point, ring and mesh architectures. One part of the optical transport hierarchy is a multiplex hierarchy, which is a hierarchy including an ordered repetition of tandem digital multiplexers that produce signals of successively higher data rates at each level of the hierarchy. Shown in FIG. 1 is an exemplary multiplexing hierarchy specified by way of optical channel data units, i.e., ODUj, where j varies from 0 to 4; and optical channel transport units, i.e., OTUk, where k varies from 1 to 4. The optical channel data units refer to a frame format for transmitting data which can be either fixed in the data rate or the data rate can be arbitrarily set.
Examples of optical channel data units that are fixed in the amount of data and data rate include those specified by ODU0, ODU1, ODU1e, ODU2, ODU2e, ODU3, ODU3e1, ODU3e2, and ODU4. An example of an optical channel data unit in which the data rate can be arbitrarily set is referred to in the art as ODUflex
One of the properties of the multiplexing hierarchy is that while the data rate changes over the different levels in the multiplexing hierarchy, the frame format can remain the same. An ODU0 frame format 10 is shown in FIG. 2. Like all other ODUjs, the ODU0 frame format 10 includes a structure of four rows and 3824 columns, as presented in FIG. 2. The ODU0 frame format 10 is further divided into an ODUk overhead area 12 (the first fourteen columns) and an optical channel payload unit (OPU) area 14. The optical channel payload unit area 14 contains two columns of overhead and 3808 columns of payload area which is available for the mapping of client data.
The nominal ODU0 rate equals half the optical channel payload unit area 14 rate of an ODU1. The latter is tailored for transport of STM-16/OC-48 signals at 2,488.32 Mbit/s. The ODU0 rate is 1,244.16 Mbit/s±20 ppm, while the rate of the available OPU0 payload area is 1,238.95431 Mbit/s.
Shown in FIG. 3 is a frame format 16 having two ODU0s multiplexed into an ODU1. The payload area of ODU1 frame format 16 of the latter has been divided into two time slots called optical channel tributary unit (or slots) 0 and 1 (ODTU01). ODTU01 is a combination of the payload area as well as a justification area which is shown as Rows 1-4 and Column 16 in FIG. 3. As shown in FIG. 3, each ODU0 is mapped into an ODTU01 time slot using a procedure known in the art as asynchronous mapping procedure (AMP), which is consistent with the legacy mapping of ODUj into ODUk.
The optical channel data units within the multiplexing hierarchy are referred to in the art as lower order or higher order. A higher order optical channel data unit refers to a server layer to which a lower order optical channel data unit (client layer) is mapped to. Optical channel data units include a parameter referred to as tributary slot granularity which refers to a data rate of the timeslots within the optical channel data unit. The tributary slot granularity of optical channel data units include time slots of approximately 1.25 Gbit/s or 2.5 Gbit/s. OPUk (when k=1, 2, 3, 4) is divided into equal sized Tributary Slots or Time Slots of granularity (1.25G or 2.5G) to allow mapping of lower order ODUj (where j<k). For example: On OPU4, there are 80 (1.25G) Tributary Slots. To map: ODU3 into OPU4=>31 TSs are used; ODU2/2e into OPU4=>8 TS are used; ODU1 into OPU4=>2 TSs are used; and ODU0 into OPU4=>1 TS is used.
ODTUG refers to grouping of ODTU entities that facilitate mapping of any ODUj into ODUk. ODTUjk refers to Optical Channel Tributary Unit j into k. This defines Tributary Slot grouping for mapping ODUj into ODUk. In particular, OPU2 and OPU3 support two tributary slot granularities: (i) 1.25 Gbps and (ii) 2.5 Gbps. Information indicative of tributary slot granularity can be encoded into the overhead of the ODUk optical channel data unit.
“Multi-stage ODU multiplexing”, refers to an optical transport network multiplexing hierarchy in which an ODUi container can first be multiplexed into a higher order ODUj container, which is then multiplexed into a higher order—ODUk container. A single-stage multiplexing refers to one lower order ODUj multiplexed into a higher order ODUk. The single stage ODU multiplexing can be heterogeneous (meaning lower order ODUj of different rates can be multiplexed into a higher order ODUk).
Optical transport networks support switching at two layers: (i) ODU Layer, i.e., time division multiplexing and (ii) OCH Layer—Lambda or wavelength switching where OCH stands for Optical Channel. The nodes on the optical transport network may support one or both the switching types. When multiple switching types are supported Multi-Layer Network (MLN) based routing as described in [RFC5339] is assumed.
Generalized Multiprotocol Label Switching includes multiple types of optical channel data unit label switched paths including protection and recovery mechanisms which specifies predefined (1) working connections within a shared mesh network having multiple nodes and communication links for transmitting data between the nodes; and (2) protecting connections specifying a different group of nodes and/or communication links for transmitting data in the event that one or more of the working connections fail. Data is initially transmitted over the optical channel data unit label switched path referred to as a working connection and then when a working connection fails, the Generalized Multiprotocol Label Switching protocol automatically activates one of the protecting connections for redirecting data within the shared mesh network.
However, the mechanisms defined in GMPLS for setting up the optical channel data unit label switched paths have overlooked a number of issues related to the multiplexing hierarchy. In particular, the present mechanisms defined in GMPLS permit the nodes to distribute TE-Link information including the maximum label switched path bandwidth in bytes/second. It might appear that this parameter alone should allow the determination of the number of units of a particular signal type (e.g. ODU0/ODU1/ODU2 etc.) a given link can support with the currently advertised cumulative unreserved bandwidth; the procedure is to divide the cumulative unreserved bandwidth by the nominal rate of the desired signal type (which is documented in G.709). There are reasons why this simple reasoning fails: (a) the OPUk (k=1/2/3/etc.) payload bandwidth is partitioned into the form of several tributary slots, with a tributary slot granularity of approximately 1.25G or 2.5G (b) the OPUk payload bandwidth is allocated in multiples of tributary slots (c) there is some bandwidth wastage due to excess capacity in the tributary slot. Thus for example, an OPU4 (on an OTU4 link) tributary slot has a nominal bandwidth of 1.301G, whereas the ODU0 bandwidth is approximately 1.24G. As such, approximately 0.06G of bandwidth is “wasted” within each tributary slot that is carrying an ODU0 signal. Over the 80 tributary slots, this amounts to a cumulative wastage of approximately 80*0.06G or 4.8G. A simple calculation of the form 80*1.301G/1.24G would suggest that 83 ODU0s can be carried within an OTU4, whereas only 80 ODU0 can be really multiplexed at the first level (equal to the number of tributary slots in OPU4). This example establishes that even on a link with a single-stage of ODU multiplexing, the cumulative bandwidth can't be used for determining the number of various signal types that can be supported, during path computation at the source node (or head-end).
Another limitation of advertising only the cumulative unreserved bandwidth in bytes/sec is as follows. This scenario happens when multi-stage ODU multiplexing is involved (either due to user choice, or due to restrictions in the hardware supporting the OTUk interfaces). Multi-stage ODU multiplexing can fragment the unreserved bandwidth into fragments which cannot be combined so as to be able to transport a signal of sufficiently large bandwidth. For example, assume that an ODU3 interface only supports the direct multiplexing of 4×ODU2 into the ODU3 container; in other words, ODU0 and ODU1 cannot be directly multiplexed into the ODU3 container (which is allowed by G.709). Here, the mapping of ODU1 and ODU0 is possible only through second stage multiplexing underneath ODU2. If two ODU1 are created under two different ODU2, only two more ODU2 can be created further on the interface although 28 Tributary Slots (1.25 Gbps) are unreserved on the interface (ODU hierarchy). This can result in unused bandwidth since all the unreserved Tributary Slots cannot be used in a concatenated fashion; such concatenation of tributary slots is possible at first level within the OPUk.
A further limitation of advertising the cumulative unreserved bandwidth mechanism appears in the context of bundled links. In bundled links, advertising only the total unreserved bandwidth does not give enough information about the distribution of the unreserved bandwidth among the component links of the bundle; the problem is compounded by the fact that OTUk links with dissimilar rates (and hence dissimilar tributary slot bandwidth granularities) are allowed to be bundled. Without knowing the distribution of unreserved bandwidth among the component links (and the tributary slot bandwidth for the component links), it is impossible to accurately evaluate the number of ODU signal of a given type that can be carried over the bundled link.
The mechanism proposed in this disclosure circumvents all these problems by advertising the exact number of ODU containers (of each ODU signal type with a fixed rate) the link is able to carry. For ODUflex containers which can have arbitrary rates, the proposed mechanism advertises the total bandwidth unreserved for this signal type.
The presently disclosed and claimed inventive concept supports ODU layer switching between the nodes in ODU switched networks to reduce the described drawbacks of the conventional GMPLS system. This can be accomplished by distributing TE-link bandwidth information indicative of number of unreserved ODU containers for each signal type within a multiplexing hierarchy of signal types supported by the OTU/ODU interfaces.